The present invention relates to an anchoring element and a stabilization device for the dynamic stabilization of vertebrae or bones using such anchoring element.
Rigid fixation and stabilization devices for the fixation of bone fractures or for the stabilization of the spinal column are known. The conventional fixation and stabilization devices often consist of two bone screws that are each anchored in a bone and/or vertebra and are connected to each other by means of a rigid rod. For example, European Patent, EP 0 483 242, describes an anchoring element, which is used in conjunction with a rigid rod as a stabilization device. Rigid systems are generally used where any relative motion of the bone parts or vertebrae to be stabilized with respect to each other is not desirable, such as for example, in the case of bone fractures or other bone defects.
A bone anchoring element in the form of a polyaxial bone screw with a screw element and a receiving part for connection to a rod is known from U.S. Pat. No. 5,474,555 ('555 patent). The '555 patent describes a screw element to be anchored in the bone which is connected to the receiving part so that there is some motion between the screw element and the receiving part. However, the '555 patent does not allow for stabilization of the bone with the possibility or option of controlled motion.
In certain clinical indications, such as damaged intervertebral disks or in the presence of an artificial intervertebral disk, it is desirable to have a stabilization device enabling the vertebrae to be stabilized and yet having some limited motion. A dynamic stabilization device of this type is known, for instance, from U.S. Pat. No. 5,733,284.
These known stabilization devices, in particular the dynamic devices, are associated with the inherent risk that the rod may exert a torque onto the anchoring element. This can eventually lead to the anchoring element loosening and/or separating from the bone, and thus the stabilizing device becoming ineffective.
FIG. 9 shows the generation of a torque M around the screw axis in a conventional stabilization device 200. In this stabilization device 200 shown therein, two bone anchoring elements 202, 202′ are connected to each other by means of a curved rod 201 with a predetermined bending elasticity. Bone anchoring elements 202, 202′ are firmly anchored in two neighboring vertebrae (not shown) by means of bone screws. FIG. 9 illustrates what occurs when the two bone anchoring elements 202, 202′ are pressed together by a force F. Due to the force F, a bending moment is exerted onto the rod, which leads to a torque M around the screw axis acting on bone anchoring elements 202, 202′. Similarly, pulling the two bone anchoring elements apart with a force F leads to a torque M in the opposite direction around the screw axis. Both of these types of forces can lead to the loosening or separation of the bone anchoring element from the bone and/or vertebrae.
It is therefore an object of the present invention to provide an anchoring element and a dynamic stabilization device for the stabilization of the bone with limited motion of neighboring vertebrae or bones, in which the anchoring element is prevented from loosening or separating during operation.